1. Field of Invention
The present invention relates to a circuit and method for automatically selecting between a high speed operating mode and a low speed operating mode in a communications receiver responsive to the mode of the incoming transmission signal.
2. Description of the Related Art
Infrared wireless data communication is a useful method for short range (in the approximate range of 0-10 meters) wireless transfer of data between electronic equipment; such as, cellular phones, computers, computer peripherals (printers, modems, keyboards, cursor control devices, etc.), electronic keys, electronic ID devices, and network equipment. Infrared wireless communication devices typically have the advantages of smaller size, lower cost, fewer regulatory requirements, and a well defined transmission coverage area as compared to radio frequency wireless technology (i.e. the zone of transmission is bounded by physical walls and therefore more useful in an office environment). In addition, infrared wireless communication has further advantages with regard to reliability, electromagnetic compatibility, multiplexing capability, easier mechanical design, and convenience to the user as compared to cable based communication technology. As a result, infrared data communication devices are useful for replacing 0-10 meter long data transfer cables between electronic devices, provided that their size and costs can be reduced to that of comparable cable technology. As examples of the type of wireless communications links that are presently in use, the Infrared Data Association (IrDA) Physical Layer Link Specification 1.1e specifies two main physical layer infrared modulation protocols.
Infrared data communications devices typically consist of transmitter and receiver components. The infrared data transmitter section consists of one or more infrared light emitting diodes (LEDs), an infrared lens, and an LED current driver. A conventional infrared data receiver typically consists of an infrared photodiode and a high gain receiver amplifier with various signal processing functions, such as automatic gain control (AGC), background current cancelling, filtering, and demodulation. For one-directional data transfer, only a transmitter at the originating end and a receiver at the answering end is required. For bi-directional communication, a receiver and transmitter at each end is required. A combined transmitter and receiver is called a transceiver.
In typical high volume applications, it is now standard practice to fabricate the receiver circuitry and transmitter driver in a single integrated circuit (IC) to produce a transceiver IC. In turn, a transceiver IC, infrared photodiode and LED along with lenses for the photodiode and LED are assembled together in a plastic molded package designed to be small in size and allow placement in the incorporating electronic device so as to have a wide angle of view (typically through an infrared window on its case). The transceiver IC is designed to digitally interface to some type of serial data communications device such as an Infrared Communication Controller (ICC), UART, USART, or a microprocessor performing the same function.
A representative example of a conventional infrared data transmitter and receiver pair is shown in FIG. 1. Infrared transmitter 10 includes LED 16 which generates a modulated infrared pulse in response to transistor 14 being driven by the data signal input at D.sub.IN. The modulated infrared signal is optically coupled to an infrared detector, such as photodiode 24 normally operated in current mode (versus voltage mode) producing an output current which is a linear analog of the optical infrared signal falling on it. The infrared pulses generated by LED 16 strike photodiode 24 causing it to conduct current responsive to the data signal input at D.sub.IN thereby generating a data signal received at D.sub.IR.
In receiver 20, the signal received at D.sub.IR is transformed into a voltage signal V.sub.IR and amplified by amplifier 26. The signal output from amplifier 26 then feeds into comparator 42 which demodulates the received signal by comparing it to a detection threshold voltage V.sub.DET in order to produce a digital output data signal at The received signal waveform will have edges with slope and will often include a superimposed noise signal. As a result, V.sub.DET is ideally placed at the center of the received signal waveform so that the output data signal has a consistent waveform width despite the slope of the received signal edges. Also, placing V.sub.DET at the center of the received signal improves the noise immunity of receiver 20 because the voltage difference between V.sub.DET and both the high and low levels of the received signal is maximized such that noise peaks are less likely to result in spurious transitions in D.sub.OUT.
The received signal, however, can vary in amplitude by several orders of magnitude due primarily to variations in the distance between transmitter 10 and receiver 20. The strength of the received signal decreases proportional to the square of the distance. Depending on the range and intensity of the infrared transmitter, the photodiode outputs signal current in the range of 5 nA to 5 mA plus DC and AC currents arising from ambient infrared sources of sunlight, incandescent and fluorescent lighting. As a consequence, the center of the received signal waveform will vary, whereas V.sub.DET must generally be maintained at a constant level. To address this problem, receivers typically include an automatic gain control (AGC) mechanism to adjust the gain responsive to the received signal amplitude. The received signal is fed to AGC peak detector 36 which amplifies the signal and drives current through diode 32 into capacitor 28 when the signal exceeds the AGC threshold voltage V.sub.AGC in order to generate a gain control signal. The gain control signal increases in response to increasing signal strength and correspondingly reduces the gain of amplifier 26 so that the amplitude of the received signal at the output of amplifier 26 remains relatively constant despite variations in received signal strength.
At a minimum, infrared receiver 20 amplifies the photodetector signal current and then level detects or demodulates the signal when it rises above the detect threshold V.sub.DET thereby producing a digital output pulse at D.sub.OUT. For improved performance, the receiver may also perform the added functions of blocking or correcting DC and low frequency AC ambient (1-300 uA) signals and Automatic Gain Control (AGC) which improves both noise immunity and minimizes output pulse width variation with signal strength.
Data can be modulated on the infrared transmitted signal by a number of well known methods. One popular method is defined by the Infrared Data Association (IrDA). IrDA Physical Layer Link Specification 1.1e specifies two main physical layer infrared modulation methods. One method is a low-speed (2 Kbp/s to 1.15 Mbp/s) on-off infrared carrier, a synchronous modulation where the presence of a pulse indicates a 0 bit and the absence of a pulse indicates a 1 bit. The second method is a high speed (4 Mb/s) synchronous Four Pulse Position Modulation (4 PPM) method in which the time position of a 125 nS infrared pulse in a 500 nS frame encodes two bits of information. Communications protocols often include a preamble, which for the 1.1e specification is sixteen repeated transmissions of a predetermined set of symbols.
The IrDA specification requires that high speed receivers be able to operate in the low speed mode as well in order to establish contact with either a transmitter operating in either low or high speed mode. To obtain high speed performance, the receiver circuit must typically have higher bandwidth capability. However, as the bandwidth response is increased to accommodate high speed performance, the amount of background noise integrated into the receiver response also increases due to the wider bandwidth spectrum. In other words, the root mean square (RMS) contribution of the background noise to the overall signal response of the receiver becomes proportionally higher.
To accommodate the higher noise level in the receiver response, the input sensitivity must be approximately 200 nA, compared to about 50 to 100 nA for low speed mode, which is accomplished by raising the V.sub.DET voltage level discussed above. However, because the detection threshold is higher for high speed mode, low speed signal transitions may be lost. Therefore, it is advantageous to limit the bandwidth response to only that required to obtain good pulse fidelity (i.e. approximately twice the transmission frequency) so that less of the background noise spectrum is amplified and the input sensitivity can be kept correspondingly lower.
Mode selection at the receiving end of the transmission is therefore typically accomplished by raising the bandwidth of the receiver response until no data response is received in order to determine the maximum speed of the communications link. In other words, the bandwidth of the receiver response is increased until the communications link fails. The link failure may be due to a limitation in the light interface, such as high ambient light levels, or performance limitations in either the transmitter or receiver circuits. In general, the bandwidth of the receiver is only adjusted at the point of low-high speed mode selection.
Many conventional receiver devices can operate in either the low-speed mode or the high speed mode. One approach is to have two separate receive paths: one path for high speed response and one path for low speed response. This approach results in additional circuit area for the two receive paths and, often, additional circuit pin outs for communication outside of the receiver device. Another approach is to have a mode select control with either a dedicated pin out or a multiplexed pin out where the mode selection is performed in the downstream communications device, such as a UART or USART, that receives the demodulated data from the receiver. This approach requires additional function to be designed into the downstream device, requires a control pin out for control communication, and generally requires additional time for response.